Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 481	$ 551
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	229	215
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 120	$ 213